This invention relates to load transducers and more particularly to a load transducer using a capacitance sensor for detecting the amount of strain on a measured member.
Strain gauges have heretofore been used for detecting loads exerted on members such as the draft links of a tractor hitch. Such systems are disclosed in the U.S. Pat. No. 3,246,701 issued Apr. 19, 1966 to Horst Schulz and the U.S. Pat. No. 3,814,188, issued June 4, 1974 to Jerome J. Ahne. However, the strain gauges heretofore used in these applications have been of the resistance type wherein a resistance wire is directly mounted to the structure which will undergo elastic deformation when the strain is applied. The resistance gauges are normally cemented to the structure with an adhesive having excellent electrical insulation and mechanical bonding qualities.
These resistance-type strain gauges have not been wholly satisfactory due to the fact that a signal-handling problem exists since the resistance change associated with the strain gauge is measured over a very minute distance and is quite small, being typically less than a 1% change. Also, the accuracy, durability, and reliability of a strain gauge of the resistance type is dependent upon environmental factors with the accuracy dropping off severely under high temperatures, and the durability being a function of the mechanical fatigue characteristics of the resistance gage material and/or the structure to which it is mounted. In addition, resistance gages, which are cemented to the loaded structure, can easily experience strains from loads other than the load desired to be measured, thus resulting in load detection errors.
Some capacitor-type strain gauges have been developed; an example of such a gauge is taught in U.S. Pat. No. 3,471,758 issued to Frank D. Werner on Oct. 7, 1969, wherein a strain carrying member forms part of a capacitor and a second member forms the other part. However, this type of gauge requires the structure to be carefully measured and prepared before measurements can be taken.